As disclosed in Bial et al U.S. Pat. No. 3,774,188 it is old in the art to sense outage of a single lamp by locating the lamp in a bridge circuit and using a semiconductor device in the secondary leg of the bridge circuit to sense the voltage excursion which occurs when the lamp burns out. While the Bial et al device operates satisfactorily in many respects, it suffers from two drawbacks which restrict its use. Firstly, the Bial et al circuit is not sensitive enough to sense outage of one of a pair of parallel connected lamps. Inasmuch as lamps in vehicles are often connected in parallel, the Bial et al circuit is unsuitable for use with such lighting systems. Secondly, the Bial et al circuit is arranged such that the relative values of a reference semiconductor device and sensing semiconductor device are critical for proper operation of the circuit. Thus, with the Bial et al circuit it is necessary to match the characteristics of the sensing semiconductor device with those of the reference semiconductor device either by locating them on a common substrate in an integrated circuit or by selecting and matching discrete components. If the reference and sensing semiconductor devices are not properly matched, inaccurate lamp outage indications may result or an actual lamp outage may not be indicated.